Antireflection structures with an exceptional low refractive index and devices containing the same

ABSTRACT

Nanoporous polymers with gyroid nanochannels can be fabricated from the self-assembly of degradable block copolymer, polystyrene-b-poly(L-lactide) (PS-PLLA), followed by the hydrolysis of PLLA blocks. A well-defined nanohybrid material with SiO 2  gyroid nanostructure in a PS matrix can be obtained using the nanoporous PS as a template for the sol-gel reaction. After subsequent UV degradation of the PS matrix, a highly porous inorganic gyroid network remains, yielding a single-component material with an exceptionally low refractive index (as low as 1.1).

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The present patent application claims the benefit of Taiwan PatentApplication Number 98122686 filed Jul. 3, 2009; and U.S. patentapplication Ser. No. 12/655,342, filed Dec. 29, 2009, the contents ofwhich are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to an antireflection structure with anexceptional low refractive index, e.g. as low as 1.1.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 7,135,523 B2 discloses a method for making a series ofnanoscale microstructures, including helical microstructures andcylindrical microstructures. This method includes the steps of: (1)forming a chiral block copolymer containing a plurality of chiral firstpolymer blocks and a second polymer blocks wherein the chiral firstpolymer blocks have a volume fraction ranging from 20 to 49%; (2)causing a phase separation in the chiral block copolymer. In a preferredembodiment, the chiral block copolymer is poly(styrene)-poly(L-lactide)(PS-PLLA) chiral block copolymer, and the copolymerization process is aliving copolymerization process which includes the following steps: (a)mixing styrene with BPO and 4-OH-TEMPO to form4-hydroxy-TEMPO-terminated polystyrene; and (2) mixing the4-hydroxy-TEMPO-terminated polystyrene with[(η₃-EDBP)Li₂]₂[(η₃-“Bu)Li(0.5Et₂O)]₂ and L-lactide in an organicsolvent preferably CH₂Cl₂ to form the poly(styrene)-poly(L-lactide)chiral block copolymer. Transmission electron microscopy (TEM) and smallX-ray scattering (SAXS) studies show that when the volume fraction ofpoly(L-lactide) is about 35-37%, nanoscale helices with a pitch of 43.8nanometers and a diameter of 34.4 nanometers were observed.

US patent publication 2004/0265548 A discloses a nanopatterned templatefor use in manufacturing nanoscale objects. The nanopatterned templatecontains a nanoporous thin film with a periodically ordered porousgeomorphology which is made from a process comprising the steps of: (a)using a block copolymerization process to prepare a block copolymercomprising first and second polymer blocks, the first and second polymerblocks being incompatible with each other; (b) forming a thin film underconditions such that the first polymer blocks form into a periodicallyordered topology; and (c) selectively degrading the first polymer blocksto cause the thin film to become a nanoporous material with aperiodically ordered porous geomorphology. In a preferred embodiment,the block copolymer is poly(styrene)-b-poly(L-lactide) (PS-PLLA) chiralblock copolymer, the first polymer is poly(L-lactide), and the secondpolymer is polystyrene. Experimental results show that the first polymerblocks can be formed into a hexagonal cylindrical geomorphology with itsaxis perpendicular to a surface of the thin film. After hydrolysis toselectively degrade the first polymer blocks, a thin film having aseries of repeated nanoscale hexagonal-cylindrical channels is obtained.

US patent publication 2006/0124467 A discloses metal nanodot arrays andfabrication methods thereof. A film of a block copolymer is deposited ona conductive substrate. The block copolymer comprises first polymer andsecond polymer blocks, wherein the first polymer blocks have aperiodically ordered morphology. The first polymer blocks areselectively degraded to form a nanopatterned template comprisingperiodically ordered nanochannels. By electroplating, metal is depositedinto the nanochannels that expose the conductive substrate, thus forminga metal nanodot array.

Rong-Ming Ho, et al. in an article entitled, “Helical Nanocompositesfrom Chiral Block Copolymer Templates”, J. AM. CHEM. SOC. 2009, 131,1356-1357, disclose a three-dimensional ordered helical nanocompositeprepared with the combination of the self-assembly of a degradable blockcopolymer and sol-gel chemistry. PS with helical nanochannels isprepared first from the self-assembly of the PS-PLLA chiral blockcopolymer after hydrolysis, and then used as template. By exploiting thenanoreactor concept, sol-gel reaction is then carried out within thetemplate so as to fabricate a helical nanocomposite. SiO₂ nanohelicescan be obtained after degradation of PS template under UV exposure.

The inventors of the invention of the present application in an article,entitled “Inorganic Gyroid with Exceptionally Low Refractive Index fromBlock Copolymer Templating”, Nano Lett. 2010, 10, 4944-5000, publishedon Internet on Nov. 3, 2010, disclose an antireflection structure ofSiO₂ gyroid having an exceptional low refractive index, e.g. as low as1.1, prepared by first forming a layer of PS-PLLA chiral block copolymerwith spin coating and solvent annealing, followed by the hydrolysis,sol-gel process, and degradation of PS template described above.

Details of the disclosures in the aforesaid US patent and patentpublication, and the aforesaid articles are incorporated herein byreference.

SUMMARY OF THE INVENTION

A primary objective of the present invention provides an antireflectionstructure with an exceptional low refractive index, e.g. as low as 1.1.

Another objective of the present invention provides a device with anantireflection structure with an exceptional low refractive index, suchas flat panel displays, solar cells, omnidirectional reflectors,light-emitting diodes, LCD backlight modules, and windows.

In order to accomplish the objectives of the present invention an antiantireflection structure constructed according to the present inventioncomprises a substrate and on a surface of the substrate a layer ofporous inorganic gyroid network. Preferably, the substrate is quartz,glass, polymer, or semiconductor. Preferably, the glass substrate is anindium tin oxide (ITO) glass substrate or carbon-coated glass substrate.Preferably, the semiconductor substrate is silicon wafer or siliconoxide substrate.

Preferably, the porous inorganic gyroid network is a ceramic oxide orceramic mixed oxide selected from the group consisting of Al, Si, Ti,Zn, Zr and Ba, which can be synthesized through a sol-gel process.Further, MgF₂ and CaF₂ with lower refractive indices which can be formedby the sol-gel process can also be used as the porous inorganic gyroidnetwork of the present invention. More preferably, the porous inorganicgyroid network is SiO₂, TiO₂, or BaTiO₃, and most preferably SiO₂.

Preferably, the layer of porous inorganic gyroid network has a thicknessof about 100 nm to about 200 nm, more preferably about 120 nm to about160 nm, and most preferably about 130 nm to about 150 nm.

The present invention also provides a process for preparing anantireflection structure with an exceptional low refractive indexcomprising the following steps:

a) coating a layer of an organic solvent solution of a block copolymerhaving first polymer blocks and second polymer blocks on a substratemodified with an organic material, wherein said first polymer isselected from the group consisting of poly(L-lactide), poly(D-lactide),poly(lactide), poly(acprolactone), and said second polymer is selectedfrom the group consisting of poly(styrene), poly(vinylpyridine), andpoly(acrylonitrile);

b) solvent annealing the resultant coating layer from step a) by placingthe coated substrate from step a) in an atmosphere containing a vapor ofnonpreferential solvent so as to form a film of the block copolymerhaving the second polymer blocks as a matrix thereof and the firstpolymer blocks having a gyroid nano structure in the matrix;

c) selectively degrading said first polymer blocks to formcorrespondingly gyroid nanochannels in the matrix of said film;

d) filling an inorganic filler into the gyroid nanochannels in thematrix of said film in a liquid mixture of a filler precursor underso-gel conditions; and

e) removing the second polymer block matrix of said layer by using anultraviolet light exposure, calcination, organic solvent, asupercritical fluid or a combination thereof to obtain an layer ofporous inorganic gyroid network on the substrate.

Preferably, the coating in step a) is spin coating, slot coating,gravure coating, or blade coating, and more preferably spin coating.Preferably, the spin coating has a spin rate of 1000-5000 rpm, and morepreferably about 1500-4000 rpm. Preferably, the organic solvent solutionhas a concentration of said block copolymer ranging from 1.5-10 wt %,and more preferably about 3 wt %. The organic solvent isdichlorobenzene, chlorobenzene, dichloromethane, toluene,tetrahydrofuran and so on, and more preferably, dichlorobenzene.

Preferably, the coating layer in step a) has a thickness of about 100 nmto about 200 nm, more preferably about 120 nm to about 160 nm, and mostpreferably about 130 nm to about 150 nm.

Preferably, the substrate is quartz, glass, polymer, or semiconductor,and more preferably the substrate is quartz, glass, or semiconductor.More preferably, the organic material used to modify the substrate ishydroxyl terminated polystyrene, hydroxyl terminatedpoly(vinylpyridine), or hydroxyl terminated poly(acrylonitrile), andmore preferably hydroxyl terminated polystyrene. Preferably, thehydroxyl terminated polystyrene has a molecular weight of 5000-10000,and more preferably about 9000.

Preferably, the process further comprises d′) aging the inorganic fillerfilled in said film under controller humidity at room temperature to 70°C. for a period of 1-6 hours, prior to step e).

Preferably, said block copolymer is poly(styrene)-poly(L-lactide)(PS-PLLA) chiral block copolymer, said first polymer blocks arepoly(L-lactide), and said second polymer blocks are polystyrene.Preferably, the volume fraction of the first polymer blocks such as PLLAin said block copolymer such as PS-PLLA is 36-50%, and more preferablyabout 40%.

Preferably, in step b) the nonpreferential solvent is dichloromethane orchloroform. In one of the preferred embodiments of the present inventionthe organic vapor is dichloromethane.

Preferably, in step c) the first polymer blocks are selected degraded byhydrolysis.

Preferably, in step e) the second polymer block matrix is removed byusing an organic solvent, for examples tetrahydrofuran (THF) or toluene.Preferably, in step e) the second polymer block matrix is removed byusing a ultraviolet light exposure, for example a wavelength of 254 nmand an intensity of 3 mW/cm².

Preferably, the inorganic filler in step d) is a ceramic oxide or mixedoxide selected from the group consisting of Al, Si, Ti, Zn, Zr and Ba,which can be synthesized through the sol-gel process. Further, MgF₂ andCaF₂ with lower refractive indices which can be formed by the sol-gelprocess can also be used as the inorganic filler of the presentinvention. More preferably, the inorganic filler is SiO₂, TiO₂, orBaTiO₃, and most preferably SiO₂.

Preferably, the filler precursor in step d) is tetraethyl orthosilicate((C₂H₅O)₄Si); titanium alkoxide, for example titanium (IV) isopropoxide;or barium hydroxide/titanium (IV) isopropoxide.

More preferably, the inorganic filler is SiO₂, and the filler precursoris tetraethyl orthosilicate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot showing variation of thickness of BCP thin-film samplesas a function of spin rate according to a process of the presentinvention.

FIG. 2 are tapping-mode SPM height images of PS-PLLA thin films withincreasing film thickness on Si wafer substrate grafted by PS—OH brushesafter solvent annealing in a dichloromethane vapor according to aprocess of the present invention, where the film thickness is labeled atthe left upper corner of each image.

FIG. 3 are Tapping-mode SPM (a) height and (b) phase images of thegyroid-forming morphology for spin-coated PS-PLLA thin film on Si wafersubstrate grafted by PS—OH brushes after solvent annealing in adichloromethane vapor according to a process of the present invention.The film thickness of the sample is 150 nm.

FIG. 4 shows normal-incident transmission spectra of gyroid-formingnanostructures (200 nm thickness) with different constituents on quartzsubstrate. (a) Quartz; (b) nanoporous gyroid SiO₂ prepared by a processof the present invention; (c) PS/SiO₂ gyroid nanohybrids; (d) PS-PLLABCP; (e) nanoporous PS template.

DETAILED DESCRIPTION OF THE INVENTION

The poly(styrene)-poly(L-lactide) (PS-PLLA) chiral block copolymer andmethod of preparing the same has been disclosed in U.S. Pat. No.7,135,523 B2, which forms nanoscale microstructures, including helicalmicrostructures and cylindrical microstructures depending on the volumefraction of PLLA. US patent publication 2004/0265548 A discloses ananopatterned template for use in manufacturing nanoscale objects,wherein a spin-coated PS-PLLA layer on a substrate is subjected tohydrolysis so that PLLA is removed to form a periodically orderednanoporous topology. Rang-Ming Ho, et al. in an article entitled,“Helical Nanocomposites from Chiral Block Copolymer Templates”, J. AM.CHEM. SOC. 2009, 131, 1356-1357, further use the nanoscalemicrostructure of OS-PLLA disclosed in U.S. Pat. No. 7,135,523 B2 toprepare a three-dimensional ordered helical nanocomposite with theincorporation of the sol-gel chemistry, so as to fabricate SiO₂nanohelices.

The inventors of the present invention are the first to prepare anantireflection structure with an exceptional low refractive index, e.g.as low as 1.1, by using surface modification to the substrate andsolvent annealing on the spin-coated PS-PLLA layer. These techniqueshelp to achieve the formation of a layer of porous inorganic gyroidnetwork on the substrate.

The following examples via experimental procedures are illustrative andare intended to demonstrate embodiments of the present invention, which,however, should not be taken to limit the embodiments of the inventionto the specific embodiments, but are for explanation and understandingonly, since numerous modifications and variations will be apparent tothose skilled persons in this art.

Experiments

Abbreviation:

L-LA: L-lactide

PS: polystyrene

PS—OH: hydroxyl terminated polystyrene

PLLA: poly(L-lactide)

PS-PLLA BCP: poly(styrene)-poly(L-lactide) chiral block copolymer

PDI: polydispersity

TEOS: tetraethyl orthosilicate

DI: deionized water

BCP: block copolymer

Synthesis of PS-PLLA BCP

The PS-PLLA BCP was prepared by a double-headed polymerization sequence.We described the synthesis of the PS-PLLA sample previously [Ho, R. M.;Chen, C. K.; Chiang, Y. W.; Ko, B. T.; Lin, C. C. Adv. Mater. 2006, 18,2355-2358]. The number-average molecular weight and the molecular weightdistribution (polydispersity) of the PS were determined by GPC. Thepolydispersity of PS-PLLA was determined by GPC and the number of L-LArepeating units was determined as a function of the number of styrenerepeating units by ¹H NMR analysis. The number-average molecular weightsof PS and PLLA, and the PDI of PS-PLLA are 34000 g mol⁻¹, 27000 g mol⁻¹and 1.26, respectively. The volume fraction of PLLA is thus calculatedto be f_(PLLA) ^(v)=0.39, by assuming that densities of PS and PLLA are1.02 and 1.248 g cm⁻³, respectively.

Preparation of Gyroid-Forming Thin Films

Quartz or Si wafer was cleaned by using isopropyl alcohol, acetonesolution, and then rinsed with deionized Water. Consequently, thesurface of substrate was modified by hydroxyl terminated polystyrene(PS—OH) with molecular weights of 9000 to increase the adhesion betweenof substrate and PS-PLLA thin films for the following hydrolysisprocess. An organic solution of PS—OH (<5%) was spin coated on thesurface of the substrate with a thickness of about several nm (about 5nm), and was annealed at 170° C. for 10 min so that PS—OH was graftedonto the surface of the substrate. Ungrafted PS—OH was removed from thesubstrate by rinsing with an organic solvent such as THF. The PS-PLLAthin film was spin-coated on a substrate modified with the PS—OH asbrushes by spin-coating from a 3 wt % chlorobenzene solution of PS-PLLAat 50° C. Spin-coated films were placed in a dichloromethane saturatedchamber at room temperature. The dichloromethane vapors swelled the thinfilms and annealed it to achieve the formation of well-definedgyroid-forming thin films.

Hydrolysis of PLLA

The PLLA blocks of the PS-PLLA thin films were removed by hydrolysis,using a 0.5M basic solution that was prepared by dissolving 2 g ofsodium hydroxide in a 40/60 (by volume) solution of methanol/water.Owing to the thin thickness, it only took about 30 minutes. We expect alonger time for hydrolysis can be used to assure that all the PLLAblocks are removed completely. After hydrolysis, the hydrolyzed sampleswere rinsed using a mixture of DI water and methanol, and then used astemplates for the following sol-gel reaction.

Sol-Gel Process

The silica precursor mixture was introduced into the PS templates byimmersing the templates in TEOS/HCl_((aq.))(0.1M)/methanol mixture(weight fraction of TEOS/HCl_((aq.))(0.1M)/methanol=10/1/25) withstirring at room temperature, and then treated under controlled humidityat 50° C. for 3 h or less for aging process. After drying, PS/SiO₂gyroid nanohybrid samples were prepared.

Degradation for PS Template

To produce the gyroid-forming SiO₂ nanostructure, the nanoporous PStemplate of the PS/SiO₂ gyroid nanohybrids was degraded by exposure toUV. The degradation was carried out under atmosphere conditions for 24 husing a UV source. The intensity of the UV source was tuned for theefficient degradation of the nanoporous PS template and did not affectthe templated texture of the inorganic gyroid-forming SiO₂ structure.Exposure was to UV with a wave length of 254 nm and an intensity of 3mW/cm². Consequently, the nanoporous gyroid SiO₂ was easily obtained onthe Quartz or Si wafer.

In addition to UV exposure, organic solvent such as THF or toluene canbe used for removal of PS template to obtain nanoporous gyroid thinfilms.

Results

In this experiment, to achieve a well-defined gyroid nanostructure withcontrolled thickness of around 150 nm (see FIG. 1 for the details ofthickness control) satisfying the dimension requirement asantireflection structure for visible light, spin-coating process forthin-film formation was carried out first so as to create thin filmswith uniform thickness. Subsequently, solvent-annealing process wasconducted to acquire the equilibrium morphology as gyroid nanostructurewith large-scale orientation. To alleviate the effect of substrate,chemically modified substrate was prepared by using polystyrene withhydroxyl chain end. A substrate with a neutral or non-preferentialwetting of the substrate for PS-PLLA could be prepared from thismodification. As shown in FIG. 2, the thin-film morphologies aredifferent to the morphology of bulk sample when the thickness of thePS-PLLA thin films is approximately smaller than 4 times of thed-spacing of the (211)_(G) planes. As a result, the equilibriummorphology as gyroid nanostructure can be obtained for samples withthickness larger than 130 nm (see FIG. 3). Although it is possible tocause surface roughing due to the solvent-annealing treatment, thevariation in thickness can be reasonably controlled by dedicated solventremoval for thick enough samples as the case examined here.

For practical applications, different coating processes, such as slotcoating and gravure coating, should be available to providecost-effective approaches for the formation of large-area coatings. Infact, a tentative test has been done by using blade coating method forthe purpose. It is noted that the formation of the thin-film sample withgyroid nanostructure is thermodynamically driven process viasolvent-annealing treatment in this work. As a result, similar resultsfor the control of morphological evolution from BCP self-assembly andtemplating can be achieved.

FIG. 4 presents the normal-incident transmission spectra of thegyroid-forming nanostructures with different constituents on quartzsubstrates. The black dashed line (a) represents the transmissivity of aquartz substrate. The nanoporous gyroid SiO₂ sample possesses thehighest transmission with respect to visible light (400 nm-800 nm), asshown in FIG. 4, line (b). Notably, the nanoporous PS template (line(e)) suffers from low transmission. We speculate that the cause for thelow transmission is attributed to the scattering of visible light inwell-defined two-phase materials with significant difference between therefractive indices of the constituents besides the absorbance of PS.Accordingly, the transmissions of the PS/SiO₂ gyroid nanohybrids (line(c)) and the PS-PLLA BCP (line (d)) are higher than that of thenanoporous PS template due to the alleviation of scattering problem. Asa result, the formation of inorganic gyroid with low refractive index(estimated to be 1.1) can be successfully achieved by BCP templates.

The invention claimed is:
 1. An antireflection structure comprising asubstrate and on a surface of the substrate a layer of porous inorganicgyroid network, wherein the substrate is quartz, glass, polymer, orsemiconductor; and the porous inorganic gyroid network is SiO₂, whereinthe layer of porous inorganic gyroid network has a thickness of about100 nm to about 200 nm, and said antireflection structure is exposed tolight when it is used.
 2. The antireflection structure of claim 1wherein the substrate is a glass substrate.
 3. The antireflectionstructure of claim 2 wherein the glass substrate is an indium tin oxide(ITO) glass substrate or carbon-coated glass substrate.
 4. Theantireflection structure of claim 1 wherein the substrate is a siliconwafer or silicon oxide substrate.
 5. The antireflection structure ofclaim 1 wherein the thickness is of about 120 nm to about 160 nm.
 6. Theantireflection structure of claim 5 wherein the thickness is of about130 nm to about 150 nm.
 7. The antireflection structure of claim 1,wherein the SiO₂ porous inorganic gyroid network is formed by a sol-gelprocess.
 8. A device having an antireflection structure defined in claim1, which is a flat panel display, solar cell, omnidirectional reflector,light-emitting diode, LCD backlight module, or a window.